In mobile communication units, multiband capability is increasingly provided in conjunction with enhanced functionality. Thus, the mobile communication unit includes a plurality of transmit/receive circuits. For switching between an antenna terminal and the plurality of transmit/receive circuits, a semiconductor switch circuit is used. To achieve downsizing and power consumption reduction of the semiconductor switch circuit, a MOSFET (metal oxide semiconductor field effect transistor) is used as a semiconductor switch element for switching between the terminals. In order for the semiconductor switch circuit to have output characteristics with low loss and low distortion, the gate electrode of the MOSFET needs to be supplied with an appropriate gate potential. To obtain an appropriate gate potential by step-up or step-down from a power supply voltage, the semiconductor switch circuit includes therein a DC-DC converter including an oscillator and a charge pump. The oscillator generates harmonic noise in the output characteristics of the semiconductor switch circuit. Thus, a low-pass filter is inserted between the DC-DC converter and the semiconductor switch element. The low-pass filter reduces harmonic noise, but impairs the switching response of the semiconductor switch circuit.